Bothteeth or Halfteeth are arguably the worst of the worst actually.
Actually, being an omnivore confers a rather significant advantage when food resources are low (such as after an extinction level event). It is usually the generalists that survive such cataclysms, not the specialists.
Consider, a Bothtooth race called "human" caused the Holocaust and wiped out hundreds of species over the centuries. No Sharptooth could do that. Heck, no hippo could do that either.
A halftooth species called "human" also created the great civilizations of the world, including all of their technology, music, art, philosophy, and other works. No hippo can do that either. It is a bit myopic to look upon a species by only considering its worst aspects.
Actually, that's why I'd like the ultimate LBT villain to be an omnivore. True, we could have sweet, cute, adorable, pretty Ruby to show the good side of the human race, so to speak, but we also need a Hitler type as the bad guy.
It actually would be a neat idea to show an evil oviraptor in a film as a malicious counterpoint to Ruby. However, I don't think that many viewers would get the implication that omnivore species = representation of humanity. After all, omnivores have been shown before (in the case of Ozzy and Strut) and that implication was not apparent.
As for the sharptooth versus herbivore decision, I would choose a (probable) omnivore called Troodon, as it was believed to be the most intelligent of the dinosaurs on the basis of its cranial structure. If I could not be an omnivore, then I would probably choose to be an apex predator. What can I say, meat is tasty. 
Actually, the special advantages of being omnivore are what made my ideal finale villain so sinister. He noticed this and decided that the dino world would only be right if omnivores ruled it, figuring that a spike in the climate could damage Sharpteeth and Flatteeth alike but that Bothteeth would be able to weather it the easiest and thus, for a "safe and secure society", they should rule it.
He was going to have Red Claw rule the Sharpteeth, Sierra rule the Flatteeth, and Ozzy rule the Egg Stealers. They were all going to be his puppets.
However, Chomper, the Gang of Five, Ruby, and Strut, among others, worked against them there.
The message of that one was that each kind had a choice of being either bad or good. That nobody was "born that way" (a little sneaky bash against a certain message from a certain group that uses that phrase.)
The interesting thing though, in my fanfic, is that the Benedict Arnolds turn out to be the Domeheads in a way, as both the ones in the Great Valley and out (i.e. the Land of Mists and the Mountains that Burn) wanted this new world order. (Note: One thinks the Domeheads are Bothteeth as some are singing Creepy Crawlies with the dinos in the Great Valley yet others are attacking Cera in the Mountains that Burn and others are chasing Egg Stealers in the Land of Mists.)
Also, I have Ruby, a Ruby boyfriend, and Ruby's family, as well as a Sharptooth/Flattooth hybrid that represent good Bothteeth. I'm no means anti-omnivore. Just showing that, of all the kinds, they ARE capable of the most cruelty. True, they can do great things, but they also can do terrible things.
I wasn't saying least advantages earlier, quite the contrary. Just that the human race is responsible for the world's evil, not the lions (Sharpteeth) or the hippos (flatteeth) or even the mongooses (Egg Stealers).
Also, I made Strut be nice too. (He just needed to get away from his brother!
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Also, the way I'd plan things, Chomper would eat Egg Stealers and Tickle Fuzzies and Hoppers and Fish. That would allow him to
avoid eating Flat Teeth. If, on the other hand, things went south with Tickle Fuzzies (which it wouldn't) or Egg Stealers, he could always revert back to eating Flat Teeth (though he'd be kicked out of the Great Valley if he did that, but, still, it's an option.
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Anyway, Chomper ate hardwater sweets in "Through the Eyes of a Spiketail" (unless I missaw something or Spike missaw something. He may be the first Bothtooth Tyrannosaurus. Way to go Chomper!
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